Among electric vehicle driving devices, especially a device directly driving a wheel is called a wheel hub motor. The wheel hub motor mentioned herein is a driving device provided near a wheel of an electric vehicle. The wheel hub motor does not have to be accommodated inside the wheel. The wheel hub motor needs to be disposed inside or near the wheel. However, the inside of the wheel or the vicinity of the wheel has a comparatively narrow space. Therefore, the wheel hub motor needs to be decreased in size.
The wheel hub motor may be categorized into a type with a speed reduction mechanism and a direct drive type without a speed reduction mechanism. The wheel hub motor with the speed reduction mechanism may easily ensure a sufficient rotational force for driving the electric vehicle at the time of starting the electric vehicle or going an uphill (ascending a slope). However, since the wheel hub motor of the type with the speed reduction mechanism transmits the rotational force to the wheel through the speed reduction mechanism, friction loss occurs in the speed reduction mechanism. In the wheel hub motor of the type with the speed reduction mechanism, a rotation speed of an output shaft of a motor is normally faster than that of the wheel. Therefore, in the wheel hub motor of the type with the speed reduction mechanism, energy loss increases due to friction loss at the speed reduction mechanism especially when the electric vehicle runs rapidly.
On the other hand, since the direct drive type wheel hub motor transmits the rotational force to the wheel without using the speed reduction mechanism, energy loss may be reduced. However, the direct drive type wheel hub motor cannot amplify the rotational force using the speed reduction mechanism. Accordingly, it is difficult for the direct drive type wheel hub motor to ensure a sufficient rotational force for driving the electric vehicle at the time of starting the electric vehicle or going an uphill. As a technique for ensuring the sufficient rotational force for driving the electric vehicle, for example, Patent Literature 1 discloses a technique which is not concerned with the wheel hub motor, but includes a speed reduction mechanism with a planetary gear mechanism, and two motors.